As early as 3000 BC the Egyptians had a calendar that recognized a year of
about 365 days with 12 months of 30 days each and 5 free days at the end of
the year. Every 4th year a 6th day was added to the 5. Caesar then
implemented the Julian calendar which deleted the free days and redistributed
them so that odd numbered months had 31 days and even numbered months had 30
days for 366 days in all. Then, in 3 out of every 4 years a day was removed
from the last month [February]. The Julian calendar started on the 1st full
moon after the winter solstice; so the year started in January instead of
March. Month's names were not changed though and so September -December
formerly the 7th-10th months were now off by two.

The Roman senate changed Quintilis to "Julius" and naturally when Augustus
succeeded Caesar he renamed Sextilis to "Augustus" but August could not be
shorter than July so a day was taken from February to add to August.

In 1582, the Gregorian calendar was devised which made 2 significant changes.
First, even-century years would not be leap years unless they were divisible
by 400. Second to get rid of the 10 extra days that had accumulated under the
Julian calendar and to get Easter closer to the spring equinox the pope
declared that the day after Thursday, October 4, would be called Friday,
October 15.

Anti-Catholic sentiment kept the English from adopting the new calendar until
1752 by which time the Julian was 11 days. The day after Wednesday, September
2, was corrected to Thursday, September 14.

The Gregorian calendar will remain "accurate" until the year 4905 when it will
be off by a day. However, if we make years divisible by 4000 non leap years,
the Gregorian calendar will be "accurate" until 20,000.